Circulatory System 



69 



monary veins deliver it into the left auricle, and the left ventricle — 

 much more strongly muscular than the right — pumps it into the main 

 arterial channels, whose distribution is essentially the same as in the 

 gill-breathing vertebrates (Fig. 71). 



EMBRYONIC AORTIC ARCHES 



In the early embryonic stages of all vertebrates is developed a series 

 of aortic arches which, in their relations to the heart, dorsal aorta, 

 and pharyngeal pouches of the embryo, are essentially similar to the 

 aortic arches of an adult fish (Fig. 70). But in lung-breathing land ani- 

 mals no gills are developed at any time, and therefore no capillaries 

 corresponding to the branchial capillaries of gill-breathers intervene 

 between ventral and dorsal aorta. In later embryonic stages of reptiles, 

 birds, and mammals — all of them being lung-breathers — this system of 

 arches undergoes great reduction and modification (Fig. 73). In adult 

 reptiles only one complete arch persists — the fourth (numbered from 

 anterior to posterior) of the several arches of the embryo. In adult 

 birds and mammals the left ventricle and the aorta are joined by only 

 a single vessel which in birds is the right member of the fourth arch 

 and in mammals is the left member of that arch (Fig. 73). The pul- 

 monary arteries are a remnant of a more posterior embryonic arch, 

 probably the sixth. 



VEINS 



Main Trunks. In the basic arrangement of the veins, as they ap- 

 pear in the embryos of vertebrates (Fig. 74), the chief trunks con- 



Fig. 70. Scheme of branchial circulation in elasmobranchs. (a) Auricle; (ao) 

 afferent branchial arteries; (ao) abdominal vein; (c) gill-clefts; (cc) common carotid; 

 (da) dorsal aorta; (ea) efferent branchial arteries; (ec) external carotid artery; (hv) 

 hepatic vein; (ic) internal carotid; (jf) jugular vein; (I) liver; (pc) postcardinal vein; 

 (sc) subclavian vein; (sv) sinus venosus; (tr) truncus arteriosus; (») ventricle; (va) 

 ventral aorta. (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," 

 Philadelphia, The Blakiston Company.) 



